You’ve just opened a Web page or clicked a link in an email when your computer’s desktop goes gray. A browser window pops up with the FBI logo in the top left corner. Below it is a live webcam feed with a picture of someone’s face. You try to click away but find that your browser is locked. With a start, you recognize the face staring at you from the screen: It’s you.

This isn’t the plot of a Japanese horror film. It’s a frightening form of malware called “ransomware” that has been seen with increasing frequency in recent months. No one knows exactly how many people have been hit with it, but security firm McAfee reports that it recorded more than 120,000 new samples in the second quarter of 2012, a fourfold increase from the same quarter last year.

There are many variants of ransomware, all of which begin by locking you out of your own machine. The next phase: trying to blackmail, intimidate or otherwise spook you into forking over cash. You probably shouldn’t do it. But it’s easy to see why a lot of people do.